A hybrid vehicle is a type of vehicle that is driven by combining at least two types of different power sources, and is often driven by an engine that obtains a torque by combusting fuel (e.g., fossil fuel such as gasoline) and an electric motor that obtains a torque by battery power. The hybrid vehicle is an environmentally friendly vehicle which adopts an engine and an electric motor as an auxiliary power source to reduce exhaust gas and enhance fuel efficiency. Research regarding hybrid vehicles has been actively conducted to meet the current demands of enhancing fuel efficiency and developing environmentally-friendly products.
Further, a hybrid vehicle obtains a driving force by a driving motor when the vehicle departs or is driven at a substantially low speed since engine efficiency is lower than motor efficiency during an initial departure of the vehicle and therefore it is advantageous to start the initial departure (vehicle starting) of the vehicle by the driving motor having efficiency greater than the engine in terms of fuel efficiency of a vehicle. A starter and generator, that is, a hybrid starter and generator (HSG) which operates an engine to provide (that is, output a cranking torque) a torque after the vehicle departs starts the engine and thus may simultaneously use an engine output and a motor output.
Accordingly, a hybrid vehicle is driven in a driving mode such as an electric vehicle (EV) mode which is a pure electric vehicle mode using the torque of the driving motor to drive the vehicle and a hybrid electric vehicle (HEV) mode which uses the torque of the driving motor as auxiliary power while using the torque of the engine as main power. In particular, the HSG starts the engine, and thus a mode conversion is performed from the EV mode to the HEV mode.
Meanwhile, the mode conversion between the EV mode and the HEV mode in the hybrid vehicle is one of the main functions, which is a factor affecting drivability, fuel efficiency, and power performance of the hybrid vehicle. A full hybrid type vehicle optimally controls an engine on/off (HEV/EV mode) to reduce fuel consumption. However, when driving resistance is increased, for example, when a substantial amount of driving power due to an uphill inclination, high-speed driving at which air resistance is increased, overloading or traction, or the like is required, the HEV/EV mode shift control which is performed while being driven under general driving conditions may not be appropriate. For example, system efficiency may be reduced due to a frequent mode shift when the vehicle is driven under a high load condition. Therefore, a special diving control method suitable for the driving under the high load condition at which the driving resistance is increased is required.